Due to environmental health concern, the conventional centralized sewage collection, treatment, and disposal systems are no longer able to fully cater to the needs of treating on-site wastewater effluent which represent a large fraction of nutrient loads and pathogen impacts. They include not only nitrogen (N) and phosphorus (P), but also pathogens such as fecal coliform and E. coli which indicate the presence of other disease-causing bacteria flowing into aquatic system to adversely affect public health.
On-site Wastewater Treatment Systems:
While residents in towns and cities are served by centralized wastewater treatment facilities, more than 25 million homes, or 25 percent of the U.S. population, still use on-site wastewater treatment systems to meet their wastewater treatment and disposal needs. this number is increasing over time ending up more than 60 million people who had to depend on decentralized systems in early 2000s in the US (US EPA, 2002). Although properly managed on-site wastewater treatment and disposal system offers several advantages over centralized wastewater treatment facilities, conventional onsite system installations might not be adequate for minimizing nitrate contamination of groundwater, removing nutrient compounds, and attenuating pathogenic organisms. The most common type of on-site wastewater treatment (OSWT) is the septic system consisting of four main components: indoor plumbing, the septic tank, the drainfield, and the soil for percolation.
This drainfield allows wastewater to percolate into the surrounding soil (the vadose zone). Concentration of ammonium and nitrate in the vadose zone of conventional septic drainfield can be decreased by understanding the physical, chemical and biological process in a drainfield. Dispersion and diffusion of fluid through solids and adsorption-desorption may be the major physical-chemical process. Biological process involves nitrification and denitrification for nitrogen species. However, systems may create a higher, undesirable level of nutrient loading if improperly designed or managed (Hoover, 2002).
Among currently available on-site wastewater treatment and disposal system treatment technologies, passive on-site wastewater treatment and disposal systems are relatively more appealing than the active counterpart because of their consistent nutrient reduction capabilities and relatively low initial and operating costs (Hossain et al., 2009; Xuan et al., 2010a, Chang et al., 2010a, 2010b, 2010c, 2010d). Passive on-site wastewater treatment and disposal system is defined by the Florida Department of Health (FDOH) as a type of onsite sewage treatment and disposal system that excludes the use of aerator pumps and includes no more than one effluent dosing pump with mechanical and moving parts and uses reactive media to assist in nitrogen removal. Reactive media are materials that effluent from a septic tank or pretreatment device passes through prior to reaching the groundwater. Some innovative technologies used one or more reactive media to assist in nitrogen removal (Smith et al., 2008).
As the nutrient impact on groundwater quality becomes a big concern, conventional on-site wastewater treatments are no longer able to fully conform to the gradually tougher water quality standards. The current major concern of on-site wastewater treatment is that a large fraction of nutrient loads, such as nitrogen and phosphorus, flow into the aquatic system and adversely affect the water quality and public health. Such a development underlines the actual requirements for a more sustainable approach in handling on-site wastewater effluent disposal.
Comly (1945) has been credited with the first recognition of the risk of nitrite and nitrate in water. Methemoglobinemia or “blue-baby” syndrome, a potentially fatal blood disorder in infants, was reported to be caused by nitrate levels over 10 mg/L. After Comly's statement became widely accepted, the U.S. Public Health Service and U.S. Environmental Protection Agency successively set 10 mg/L of nitrate nitrogen and 1 mg/L of nitrite nitrogen as an upper limit for the safety of drinking water. However, there have been a number of reported cases of methemoglobinemia caused by nitrate at less than 10 ppm in drinking water. Besides the obvious cyanosis, there are a number of serious long-term, chronic impacts following exposure to high nitrate drinking water ranging from hypertrophy of the thyroid to cancer.
Elevated nutrient and pathogen levels in groundwater may cause health problems in children and may impair or destroy environmentally sensitive habitat. Increased nutrient and pathogen concentrations in surface waters may also lead to excess plant and algal growth and water pollution. When plants and algae die and decay, it results in lower dissolved oxygen levels and overall water quality.
The transition of nitrogen from one phase to another is commonly referred to as the nitrogen cycle. Ammonia combines with organic materials to create ammonium (NH4+). In the presence of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), ammonium is converted to nitrite (NO2-) and further to nitrate (NO3-). These two reactions are collectively called nitrification. Denitrification, conversely, performed by denitrifying community, is an anaerobic respiration process using nitrate as a final electron acceptor and result in stepwise microbiological reduction of nitrate, nitrite, nitric oxide (NO), and nitrous oxide (N2O) to nitrogen gas (N2). Nitrate removal rates are directly influenced by the slow growing bacteria that govern nitrification and denitrification. Optimal temperature for the denitrifiers was found to be between 10° C. and 25° C.
Phosphorus may be removed in either an aerobic or anaerobic environment and is removed through sorption onto soil media. To remove nitrogen (N) and phosphorus (P), a wide range of alternative on-site wastewater treatment systems were developed. Aerobic treatment units are designed to treat wastewater rather than the conventional method with septic tanks alone. An aeration chamber is the most important compartment in aerobic treatment units where a pump supplying a constant flow of air and a stirring mechanism are used to oxygenate the water, creating optimum conditions for aerobic organisms to decompose organic compounds. The application of the aerobic treatment units may significantly reduce the health risk.
The main disadvantages associated with aerobic treatment units are the need for external power source and higher maintenance level required to ensure proper system operation. Sand filters in conjunction with a septic tank or an aerobic treatment unit are an alternative that is commonly used to provide additional treatment for effluent before it is discharged. The main function of sand filter is to reduce the amount of suspended solids and dissolved organic material present in the water. Microorganisms attached to the sand particles are able to aerobically digest the organic material within the wastewater. Havard et al. (2008) used six lateral flow filters (LFSFs) for their treatment of septic tank effluent. They evaluated the effects of slope and sand characteristics based on satisfactory performance of LFSFs: biological oxygen demand (BOD) (98.5%), total suspended solids (95.5%), and E coli (5.4 log reduction). Phosphorus removal ranged from 98% in the fine sand to 71.2% in the coarse sand filter. TN removal ranged from 60 to 66%.
However, owners need to periodically rake and replace clogged surface sand. regardless of the disadvantages of each of these two on-site wastewater treatment alternatives mentioned above, it can be seen that denitrification in these two alternatives does not come up to expectations due to the presence of aerobic environment. To date, the United States Environmental Protection Agency and numerous states are imposing stricter standards for the release of TN (as a combined measurement of ammonia-N and nitrite-N and nitrate-N), phosphorus and pathogenic bacteria (normally coliforms) released by septic systems to conventional leach fields. Hence, there is an urgent need to find a more effective unit operation to help septic tank system meet the upcoming USEPA regulations.
Engineered, functionalized, and natural sorption media can be used to treat stormwater runoff, wastewater effluents, groundwater flows, landfill leachate and sources of drinking water for nutrient removal via physicochemical and microbiological processes (Chang et al., 2010b, 2010c). With such functionality, the biofilm can be formed on the surface of soil particles to allow microbes to assimilate nitrogen species although nitrogen cannot be removed by sorption directly. It is indicative that sorption provides an amenable environment for subsequent nitrification and denitrification. In the progress of media development, the media section and application is no longer only limited to the common natural mineral, such as sand, limestone, expanded clay, zeolite, pumice, bentonite, oyster shell. The media may also include a variety of industrial, domestic wastes people used to consider to be: sawdust, peat, compost, wheat straw, newspaper, wood chips, wood fibers, mulch, glass, ash, tire crumb, expanded shale, and soy meal hull (Chang et al., 2010b, 2010c). Last but not the least, the choice of media mixes depends on the desired length of service, residence time during an operating cycle, and pollutants in the wastewater. In many cases, the object to be moved is not only the nutrients, but also some other pollutants, such heavy metals, pathogens, pesticides, and toxins (TCE, PAH, etc.) (Chang et al., 2010b, 2010c).
Wetland has been playing an important role in water conservation, climate regulation, soil erosion control, flood storage for use in drought, environment purification, etc. Based on the same principle for wastewater purification by natural wetlands, the man-made constructed wetland with effective management can strengthen its ability to improve the effluent water quality. The wetland system removes nitrogen in the water through a variety of mechanisms including biological, physical and chemical reactions. Its' biological functions such as ammonification, nitrification-denitrification and plant uptake under appropriate conditions are regarded as the core players for nitrogen removal. Precipitation of particular form of phosphorus is the main path for phosphorus removal. Constructed wetland, an effective small-scale wastewater treatment system with low energy, maintenance requirements and operational costs has been widely used to treat various kinds of wastewater throughout the world.
While the constructed wetland showed its remarkable removal efficiency of organic matter, nutrient, pathogen, etc, the increased stricter water quality standard motivated many more advanced studies with regard to higher commercial, aesthetic, habitat and sustainable value. The constructed wetland can be divided into three main types according to the different hydrologic modes: free water surface (FWS) wetland, horizontal subsurface flow (HSSF) wetland and vertical flow (VF) wetland. FWS wetland includes emergent vegetation, soil or medium to support the emergent vegetation, and a water surface above the substrate. In the HSSF, the wastewater is fed in the inlet and passes the filter medium under the surface until it reaches the outlet zone through the subsurface pathways. Vertical flow constructed wetland generally consists of a gravel layer at the bottom topped with a sand layer. When intermittently feeding with a large batch, the wastewater percolates vertically until it reaches a drainage network. With a variety of natural wetland systems are used successfully at the field scale, the designed models of constructed wetland with systems dynamics characteristics have gradually gained growing attention during the past decades.
Within the constructed wetland, the transition of nitrogen from one phase to another is commonly referred to as the nitrogen cycle. Ammonia combines with organic materials to create ammonium (NH4+). In the presence of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), ammonium is converted to nitrite (NO2) and further to nitrate (NO3−). These two reactions are collectively called nitrification. Denitrification, conversely, performed by denitrifying community, is an anaerobic respiration process using nitrate as a final electron acceptor and results in stepwise microbiological reduction of nitrate, nitrite, nitric oxide (NO), nitrous oxide (N2O) to nitrogen gas (N2). Nitrate removal rates are directly influenced by the slow growing bacteria that govern nitrification and denitrification.
The term, Residence Time Distribution (RTD), characterizing chemical reactors was first proposed by Danckwerts in 1953, which was oftentimes used to discuss the type of mixing in constructed wetland). The RTD function is generally measured by injecting an impulse of tracer and measuring the tracer concentration as a function of time at interior wetland points as well as the outlet. Many wetland systems were modeled as a number, N, of stirred tank reactors in series.
                    RTD        =                              N                                          (                                  N                  -                  1                                )                            !                                ⁢                                    (                              N                ⁢                                  t                  τ                                            )                                      N              -              1                                ⁢                      exp            ⁡                          (                                                -                  N                                ⁢                                  t                  τ                                            )                                                          (        1        )            Equation (1) can be considered as single continuous stirred tank reactor (CSTR) when N=1 and the plug flow reactor (PFR), N=∞.
In the earlier stage of designing the treatment model, the constructed wetlands were considered as a “black box”. Scientists focused on the influent and effluent concentration and fit the result with the designed linear or power equation to build the relationship between them. Table 1 shows the regressions for wetland nitrogen and phosphorus removal of SSF wetlands. This kind of model oversimplified the constructed wetland treatment system, which has an extremely complicated physical, chemical and biological process. Not only the influent concentration and hydraulic retention time (HRT) but also hydrodynamic conditions, such as wetland dimension, porosity and conductivity of media can affect the removal efficiency of pollutants of concern. Gradually, the first-order kinetics equation or Monod type equations were widely accepted and applied to replace the regression method. Kadlec and Knight (1996) had summarized the nitrogen removal equations of modeling the constructed wetland in Table 2 which shows the parameter and corresponding formula of modeling the constructed wetland.
TABLE 1ParametersRegression equationsOrganic NC2 = 0.1C1 + 1.0Ammonium NC2 = 0.46C1 + 3.3Nitrate NC2 = 0.62C1Total Kjeldahl Nitrogen (TKN)C2 = 0.752C1 0.821 q0.076Total nitrogen (TN)C2 = 0.46C1 + 0.124q + 2.6PhosphorusC2 = 0.51C11.10Note: C2=concentration at the outlet;C1=concentration at the inlet;q=hydraulic loading rate
TABLE 2ParameterFormulaNitrifier growth rate (uNITR)      u    NITR    =      172    ⁢                  ⅇ                  0.098          ⁢                      (                          T              -              15                        )                              ⁡              [                  1          -                      0.833            ⁢                          (                              7.2                -                pH                            )                                      ]              ⁢          (                        C          AN                          1          +                      C            AN                              )        ⁢          (                        C          DO                          1.3          +                      C            DO                              )       Denitrifier growth rate (UDENITR)      u    DENITR    =            u      DENITRmax        ⁡          [                        (                                    C              NN                                                      K                DENITR                            +                              C                NN                                              )                ⁢                  (                                    C              ORGC                                                      K                ORGC                            +                              C                ORGC                                              )                    ]       Outlet concentration of ammonium nitrogen (CAN)      C    AN    =                    (                  C          ANi                )            ⁢              ⅇ                              -            kAN                    /                      (                          Q              /              A                        )                                +                  (                              k            ON                                              k              AN                        -                          k              ON                                      )            ⁢              (                              C            ONi                    -                      C            ON            *                          )            ⁢              (                              ⅇ                                          -                                  k                  ON                                            /                              (                                  Q                  /                  A                                )                                              -                      ⅇ                                          -                                  k                  AN                                            /                              (                                  Q                  /                  A                                )                                                    )            Note:                KDENITRN=denitrification half-saturation constant, mg/L        KORGC=organic nitrogen half saturation constant, mg/L        kON, kAN=first-order organic nitrogen, ammonium loss rate, g/m2/yr        CAN, CDO, CNN, CORGC, CON=concentration of ammonium, dissolved oxygen,        nitrite+nitrate, organic carbon, organic nitrogen, mg/L        CANi, CONi=inlet concentration of ammonium nitrogen, organic nitrogen, mg/L        CON*=background concentration of organic nitrogen, mg/L        Q/A=hydraulic loading rate        
Tunçsiper, B., Ayaz, S. C., Akça., L., (2006), Modelling and evaluation of nitrogen removal performance in subsurface flow and free water surface constructed wetlands. Water Science & Technology, 53 (12), 111-120 simulated removal efficiencies of nitrogenous pollutants in SSF and FWS constructed wetland systems. Two types of the models, first-order plug flow and multiple regressions, were used to evaluate the system performances. Nitrification, denitrification and ammonification rate constants values in SSF and FWS systems were 0.898 d−1, 0.486 d−1 and 0.986 d−1, and 0.541 d−1, 0.502 d−1, and 0.908 d−1, respectively. They found that the first-order plug flow model clearly estimated slightly higher or lower values than observed when compared with the other models.
Jou, C. U., Chen, S. W., Kao, C. M., Lee, C. L. (2008), assessed the efficiency of a constructed wetland using a first-order biokinetic model. Wetlands, 28(1), 215-219 tried a constructed wetland for restoring a creek. The ecological treatment system removed 64.0% of suspended solids (SS), 43.0% of biochemical oxygen demand (BOD), and 11.0% of ammonia nitrogen. A first-order biokinetic model was used to estimate the reductions of BOD and nitrogenous biochemical oxygen demand (NBOD). They reported that the first-order biokinetic model appeared useful for estimating BOD and NBOD reductions in a constructed wetland. However, the fatal limitation of the first-order kinetics is that the constructed wetland system is required to keep the same flow rate, concentration and ideal plug flow.
To make the dynamic modeling of the constructed wetland processes more acceptable and flexible, Pastor, R., Benqlilou, C., Paz, D., Cardenas, G., Espuna, A., Puigjaner, L., (2003), Design optimization of constructed wetlands for wastewater treatment. Resources, Conservation and Recycling, 37(3), 193-204 proposed the design optimization of constructed wetland for wastewater treatment by combining a first principal model and an artificial neural network, which had a main advantage for better representing highly non-linear multi-input/multi-output system.
Tomenko, V., Ahmed, S., Popov, V., (2007), Modelling constructed wetland treatment system performance Ecological Modelling, 205, 355-364 compared multiple regression analysis (MRA) and two artificial neural networks—multilayer perception (MLP) and radial basis function network (RBF) in terms of their accuracy and efficiency when applied to prediction of the biochemical oxygen demand (BOD) concentration at effluent and intermediate points of subsurface flow constructed wetlands. The dataset was normalized and transformed using principal component analysis (PCA) to increase the efficiency of the modeling. Artificial neural networks models were eventually cross-validated to find optimal network architectures and values of training algorithm parameters.
The models mentioned above just provide a limited understanding of specific items, which were even separately analyzed. The mechanistic approach for modeling constructed wetland systems has been highly regarded by people who prefer to understand the mystery of the whole wetland treatment process. Wynn, T. M., Liehr, S. K., (2001), Development of a constructed subsurface flow wetland simulation model, J. of Ecological Engineering. 16, 519-536 used a mechanistic compartmental simulation model, which included six linked submodels: the carbon cycle, the nitrogen cycle, a water balance, an oxygen balance, autotrophic bacteria growth and heterotrophic bacteria growth. Darcy's law was used to describe the flow through the media. The wetland was regarded as either a single continuously stirred tank reactor or a series of continuously stirred tank reactors instead of plug flow reactors, which was considered to be a better reactor model for simulating non-ideal plug flow. Monod kinetics was utilized to describe microbial growth rate. Transformations, such as nitrification and denitrification, were then linked directly to microbial growth. In general, except for the oxygen, the result of effluent BOD, organic nitrogen, ammonium and nitrate concentration fit the model well.
Langergraber G., (2001), Development of a simulation tool for subsurface flow constructed wetlands, Wiener Mitteilungen. 169, 207 presented a multi-component reactive transport module CW2D to model the biochemical transformation and degradation processes in SSF CWs. The mathematical structure of CW2D was based on that of the ASMs (Henze et al., 2000). The CW2D consisted of twelve components, nine process and forty-six parameters. The HYDRUS-2D was incorporated by using Richards equation to describe the variably saturated water flow conditions. Water uptake by plant roots was accounted as a sink term in the flow equation.
The components considered ammonium, nitrite, nitrate and nitrogen gas; dissolved oxygen; organic matter; inorganic phosphorus; heterotrophic and two species of autotrophic microorganisms. The rates of the biochemical elimination and transformation processes were described by using Monod-type of equation. Recently, Giraldi et al. (2009a, b) developed a mathematical model (FITOVERT) to analyze the hydrodynamics of a one-dimensional vertical flow CW under three different saturation conditions: complete saturation, partial saturation, and complete drainage by dosing rhodamine WT in steady state conditions. Richards equation was used for modeling the variably saturated conditions, while van Genuchten-Mualem functions was used to describe the relationships between pressure head, hydraulic conductivity, and water content. In particular, the porosity reduction due to bacteria growth and accumulation of particulate component (i.e. clogging process) can be simulated by FITOVERT.
When researchers revel in improving CWs, the complexity of the latest generation model insensibly deters the public prevalence of the CW model application. Massive complicated partial differential equation let the CW engineer flinch, which ties up the development of CW design and operation. The limited useful results from real practice further retard the calibration and optimization of the theoretical modeling work. To break this vicious cycle, some intuitive and accessible model should be developed to fit the gap. The objective of this research is to develop a simplified compartmental dynamics simulation model of subsurface upflow wetlands (SUW) to provide a dependable reference and tool for design of a subsurface upflow wetland, a competitive candidate of on-site wastewater treatment technologies.